Azure App Service separates the hosted application and the resources it uses by using a concept called App Service Plan. The App Service plan defines the computing resources (virtual machines) that host your applications, and multiple applications can be hosted on an App Service plan.

These are the things that an App Service plan defines:

• Region: the location of the computing resources; for example, West US, East Europe, and so on
• The amount of VM instances 
• The size of VM instances (small, medium, large)
• Pricing tier: determines the features you can use and how much you pay (Free, Shared, Basic, and so on)